The present invention relates to a curable composition for tendons for prestressed concrete, and more particularly to a curable composition which is applied to the surface of a tendon used in a post-tensioning system for prestressed concrete for the purpose of preventing the tendon from being corroded or rusted and obtaining good bond characteristic between the tendon and concrete. The present invention also relates to a sheath-covered tendon for prestressed concrete, which is not required to inject a grout into a sheath in a post-tensioning system for prestressed concrete, is bonded with concrete after tensioning the tendon and is completely prevented from being corroded.
A prestressed concrete technique is a technique that steel such as a piano wire is arranged for applying compression force in advance to a place, on which a tensile load acts, and is tensioned when the strength of concrete reaches a certain level. The prestressed concrete technique includes a post-tensioning system and a pretensioning system.
In the conventional post-tensioning system, a metal-made sheath is buried in concrete before concrete placing, a tendon such as prestressing steel (steel wire for PC, steel strand for PC, steel bar for PC or the like) is inserted into this sheath, and the tendon is tensioned after hardening the concrete. Lastly, a grout such as a cement milk is injected between the sheath and the tendon for preventing the tendon from being corroded and obtaining good bond characteristic between the tendon and the concrete. In this system, however, the injecting work of the grout such as the cement milk is complicated and forms the main cause of increase in cost. In addition, this system has involved such problems that the injection is liable to become incomplete, and so the tendon may gather rust.
In order to solve such defects, therefore, for example, Japanese Patent Publication No. 69939/1993 has proposed a tendon for prestressed concrete, to the surface of which a resin, with which a curing agent has been mixed at a mixing ratio according to the prescribed curing time so as not to cure before the tendon is tensioned and so as to cure at normal temperature after the tendon is tensioned, to initiate curing, has been applied. In Examples of this publication, curable compositions with a tertiary amine curing agent containing a curing accelerator added to an epoxy resin are disclosed.
Japanese Patent Publication No. 11791/1996 has proposed that a curable composition comprising an epoxy resin as a main component and a latent curing agent that undergoes chemical curing at normal temperature, such as dicyandiamide, is provided as a coating material for tendons for prestressed concrete.
According to the method making use of these curable compositions, the tendon can be completely prevented from being corroded without conducting grouting. More specifically, in this method, the epoxy resin (curable composition), in which the kind and amount of the curing agent have been adjusted so as not to cure before the tendon is tensioned and so as to cure at normal temperature after the tendon is tensioned, is applied to the tendon, and the tendon is tensioned after the concrete has hardened and before the epoxy resin cures. At this time, the tensioning is feasible because the epoxy resin is liquid. After the tensioning, the epoxy resin gradually cures at normal temperature to finally bond the tendon with the concrete and completely prevent it from being corroded. After applying the epoxy resin to the tendon, the coated surface may be covered with a resin sheath as needed. In this case, the tendon is bonded with the concrete through the resin sheath.
However, the conventional curable compositions have involved the following problems. Concrete generally generates heat upon curing after the concrete is placed. Therefore, the concrete structure may be heated to a high temperature near 100xc2x0 C. in some cases according to the size and shape thereof. The conventional curable compositions are stable at room temperature for a long period of time, but quickly undergo a reaction at a high temperature near 100xc2x0 C. It is extremely difficult to control the formulation of a curable composition containing an epoxy resin so as not to cure at a high temperature near 100xc2x0 C. and so as to retain the curability at normal temperature.
When the curable composition applied to the tendon is prematurely cured by the heat generated upon the hardening of the concrete, the tendon can not be tensioned after the concrete has been hardened. On the other hand, when the amounts of the curing agent and curing accelerator are lessened, the premature curing of the curable composition at the high temperature can be prevented. However, the curing period at normal temperature extremely lengthens. Accordingly, the technique by the post-tensioning system for prestressed concrete using the conventional curable compositions has involved a problem that applicable places and objects are limited.
It is an object of the present invention to provide a curable composition for tendons for prestressed concrete, which can completely prevent the tendons from being corroded without conducting injection of a grout into a sheath and used even when it is heated to a high temperature by heat generated upon hardening of concrete.
Another object of the present invention is to provide a sheath-covered tendon for prestressed concrete, which is not required to inject a grout into a sheath in a post-tensioning system for prestressed concrete, is bonded with concrete after tensioning the tendon and is completely prevented from being corroded.
The present inventors have carried out an extensive investigation with a view toward achieving the above-described objects. As a result, it has been found that a curable composition containing at least an epoxy resin and a moisture-curing type curing agent, wherein the tensioning-permitting time at 90xc2x0 C. and the number of days required to cure at normal temperature of the curable composition satisfy the respective specified relationships, is suitable for tendons for prestressed concrete.
When said curable composition is applied to the surface of a tendon, and the tendon is used in a technique for prestressed concrete according to a post-tensioning system, the curable composition does not prematurely cure even when it is heated to a high temperature by heat generated upon hardening of concrete, and so the tensioning of the tendon is feasible. In addition, the curable composition undergoes a curing reaction at normal temperature thereafter, whereby the tendon can be bonded with the concrete directly or through a sheath. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is provided a curable composition for tendons for prestressed concrete, which is applied to the surface of a tendon for prestressed concrete, said curable composition comprising at least an epoxy resin and a moisture-curing type curing agent, wherein the tensioning-permitting time L at 90xc2x0 C., which is expressed by the time required until the viscosity of the curable composition as measured at 25xc2x0 C. at the time when the curable composition has been left to stand under environment of 90xc2x0 C. and 60% in relative humidity reaches 10,000 poises, and the number of days M required to cure at normal temperature, which is expressed by days required until the type D Durometer hardness of the curable composition as measured in accordance with JIS K 7215 at the time when the curable composition has been left to stand at 25xc2x0 C. reaches 60, satisfy the respective relationships represented by the following expressions (1) and (2):
L (hours)xe2x89xa720xe2x80x83xe2x80x83(1) 
M (days)xe2x89xa61095xe2x80x83xe2x80x83(2). 
According to the present invention, there is also provided a sheath-covered tendon for prestressed concrete having a structure that a curable composition is applied to the surface of a tendon for prestressed concrete, and the coated surface thereof is covered with a sheath, wherein the curable composition comprises at least an epoxy resin and a moisture-curing type curing agent, wherein the tensioning-permitting time L at 90xc2x0 C., which is expressed by the time required until the viscosity of the curable composition as measured at 25xc2x0 C. at the time when the curable composition has been left to stand under environment of 90xc2x0 C. and 60% in relative humidity reaches 10,000 poises, and the number of days M required to cure at normal temperature, which is expressed by days required until the type D Durometer hardness of the curable composition as measured in accordance with JIS K 7215 at the time when the curable composition has been left to stand at 25xc2x0 C. reaches 60, satisfy the respective relationships represented by the following expressions (1) and (2):
L (hours)xe2x89xa720xe2x80x83xe2x80x83(1) 
M (days)xe2x89xa61095xe2x80x83xe2x80x83(2). 